1. Field of the Invention
The present invention relates to the field of optical devices such as photon detectors and optical memory structures. More specifically, the present invention relates to photon detectors which may be configured to detect single photons and also on optically programmable non-volatile memory.
2. Discussion of the Background
There is a need for an optical detector which is capable of detecting a single photon. Recently, this need has been heightened by the advent of quantum cryptography of optical signals. In essence, quantum cryptography relies upon the transmission of data bits as single particles, in this case, photons, which are indivisible. One way in which the data can be encoded is via the polarisation of the electric field vector of the photons. The key component of such a system is a detector which can respond to individual photons. It has been proposed that quantum cryptography can be used to transmit the key for the encryption of data.
Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. An optimum signal to noise ratio is achieved when a single photon is detected, as the noise is then limited by the shot noise and is independent of noise arising due to the detector amplifier.
A single photon detector could also be used for time-of-flight ranging experiments where the distance-of an object from a fixed point is measured by calculating the time over which a single photon takes to return to a detector. This technique can also be used to scan the surface of an object, even a distant object, to form a spatial image of its surface depth, thickness etc.
Single photon detectors are available in the form of photo multiplier tubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs have the disadvantage of having low quantum efficiency, being expensive, bulky, mechanically fragile, requiring high biasing voltages and cooling. They can also be damaged and can require a long settling time after exposure to high light levels or stray magnetic fields. On the other hand, SPADs have the disadvantage of having a relatively low gain and high dark count rates, especially when operated at higher repetition rates. They are also expensive and require high bias voltages and external cooling.